This discussion about United often narrows down to size of fanbase or shirts sold. While I agree that is one of the main metrics of club stature, there are other main metrics as well, including: 1) Trophy haul (domestic league, European, domestic cup), peak level/unique accomplishments, list of legends.
RM, Barca, Bayern, Pool (arguable), and AC Milan are superior to United in terms of trophy haul.
RM, Barca, Bayern, Pool. AC Milan are superior in terms of peak level/unique accomplishments (50s RM, Pep's Barca and the broader 06-15 dominance, Sacchi's Milan, and RM 3-peat team are unique and huge contributions to football history/lore). United do have the 98-99 treble so they're not slouches here but it's still a tad lower than those amazing accomplishments. Bayern had the European cup 3 peat and Pool had 4 CLs and 6-7 league titles in 9 years in 70s and 80s.
United are definitely behind RM, Barca, and AC Milan in list of legends/pedigree of players who've played for them. They're in the mix with Bayern and ahead of Pool in this regard.
So, basically, if you look at the entirety of the metrics, it's simply not possible to consider United the biggest club in the world.